1. Field of the Invention
This invention relates to liquid level detectors, and more particularly to an apparatus for sensing the level of a medium using a thermoresistive sensor.
2. Description of the Related Art
Prior art liquid level detectors, such as fuel detectors for motor vehicles, usually include a float that rides on an upper surface of the fuel in a fuel tank. The float is typically connected to one end of a pivot arm while the other end of the pivot arm typically includes a wiper mechanism that brushes against a resistor strip when the arm is rotated due to a change in fuel level in the tank. The wiper mechanism, when rotated, causes a change in electrical resistance that can be displayed at a convenient location in the passenger compartment of the vehicle in order to indicate fuel level within the tank. Liquid level gauges of this type are subject to wear due to the number of interconnected moving parts. Worn parts may result in complete mechanical breakdown of the gauge or, at the very least, inaccurate liquid level detection.
Prior art attempts to overcome the inherent deficiencies in float-type gauges have included sensor elements of the thermoresistive or thermistor type, wherein the thermal characteristics of fluids are used to monitor the level of liquid in a tank. These gauges generally include electrically heated thermistors located within the tank. The temperature of the thermistors and, consequently, their electrical resistance, increase as the liquid in which the thermistors are immersed, decreases. This change in resistance is used to determine the level of liquid in the tank. Generally, one thermoresistive element is used for measurement while a second thermoresistive element is used to compensate for errors which can occur as a result of temperature changes of the liquid itself.
U.S. Pat. No. 5,685,194 to McCulloch et al., the disclosure of which is hereby incorporated by reference, discloses a liquid level gauge having first and second thermoresistive sensors. Separate constant current sources supply power to each sensor. One of the current sources supplies sufficient current to the first sensor to cause self-heating of the first sensor, while the other of the current sources supplies relatively little current to the second sensor. Level or flow is determined by dividing the difference in voltages across the first and second sensors by the voltage across the second sensor. Other resistance-type liquid level sensors are disclosed in U.S. Pat. No. 3,600,946 and U.S. Pat. No. 4,805,454, the disclosures of which are hereby incorporated by reference.